A convective thermal blanket for preoperative therapeutic and comfort warming includes an inactive surface region adapted to lie against the anterior thoracic and/or abdominal area of a person extending at least from the thighs to the abdomen of the person and active surface regions adapted to circulate warmed air to limbs of the person.
Use of the term “convective” to denote the transfer of heat by the circulation of warmed air from a convective thermal blanket to a body refers to the principal mode of heat transfer, it being understood that heat may at the same time be transferred between a thermal blanket and a body by conduction and radiation, although not to the degree of convection.
Convective thermal blankets that transfer heat to a human body are known. For example, there are blankets that receive a stream of pressurized, warmed air, inflate in response to the pressurized air, distribute the warmed air within a pneumatic structure, and emit the warmed air onto a body to accomplish such objectives as increasing comfort, reducing shivering, and treating or preventing hypothermia. These blankets are typically called “convective thermal blankets” or “covers”; for convenience, in this application they shall be called, simply, “thermal blankets.” Arizant Healthcare Inc., the assignee of this application, makes and sells such blankets under the BAIR HUGGER® brand. One such blanket is the Model 522 Upper Body Blanket.
The term “perioperative” is defined in the PDR Medical Dictionary, Second Edition, (Medical Economics Company, 2000), as “around the time of operation.” The perioperative period is characterized by a sequence including the time preceding an operation when a patient is being prepared for surgery (“the preoperative period”), followed by the time spent in surgery (“the intraoperative period”), and by the time following an operation when the patient is closely monitored for complications while recovering from the effects of anesthesia (“the postoperative period”).
According to Mahoney et al. (Maintaining intraoperative normothermia: A meta-analysis of outcomes with costs. AANA Journal. 4/99; 67, 2:155-164.), therapeutic warming is employed during at least the intraoperative period in order to prevent or mitigate a constellation of effects that result from hypothermia. In fact, it is increasingly manifest that maintenance of normothermia perioperatively enhances the prospects for a quick, successful recovery from surgery. The effectiveness of therapeutic warming depends upon delivery of enough heat to a patient's body to raise the patient's core body temperature to, or maintain it within, a narrow range, typically near 37° C. This range is called “normothermic” and a body with a core temperature in this range is at “normothermia.” Hypothermia occurs when the core body temperature falls below 36° C.; mild hypothermia occurs when core body temperature is in the range of 34° C. to 36° C. Therefore, “perioperative therapeutic warming” is warming therapy capable of being delivered during one or more of the perioperative periods for the prevention or treatment of hypothermia.
Therapeutic warming is contrasted with “comfort warming” which is intended to maintain or enhance a patient's sense of “thermal comfort.” Of course, therapeutic warming may also comfort a patient by alleviating shivering or a feeling of being cold, but this is a secondary or ancillary effect. Thermal comfort is a subjective notion; however, the environmental conditions necessary to produce a sense of thermal comfort in a population of human beings are known and well tabulated. For example, Fanger (Thermal Comfort: Analysis and Applications of Environmental Engineering. Danish Technical press, Copenhagen, 1970) defines thermal comfort as “that condition of mind which expresses satisfaction with the thermal environment.” Even when a patient is normothermic, less than ideal conditions can result in acute feelings of discomfort. Under normothermic conditions, thermal comfort is largely determined with reference to skin temperature, not core body temperature. Comfort warming is warming applied to a patient to alleviate the patient's sense of thermal discomfort.
Therapeutic warming may be indicated during any one or more of the perioperative periods. For example, for a short operation in a surgery with no warming equipment available, a person may be warmed preoperatively in a preparation area to raise mean body temperature to a level higher than normal in order to store enough thermal energy to maintain normothermia, without heating, intraoperatively. After surgery, it may be necessary to apply therapeutic warming in a recovery area to raise the core temperature to normothermia and maintain it there for a period of time while anesthesia wears off. Alternatively, for a long surgery in an arena with heating equipment available, a person may be warmed for comfort before surgery and warmed therapeutically during and after surgery.
Therapeutic warming may be provided by a convective thermal blanket that receives and distributes warmed, pressurized air in an inflatable pneumatic structure and then expels the distributed air through one or more surfaces toward a patient in order to prevent or treat hypothermia in the patient. An example of such use is found in U.S. Pat. No. 6,524,332, “System and Method for Warming a Person to Prevent or Treat Hypothermia”, commonly owned with this application. Comfort warming by convective means is described in the referenced U.S. Patent Application, and the referenced Publication No. WO 03/086500.
When delivered by convective devices, therapeutic warming is distinguished from comfort warming by intended effects and by the parameters of heat delivery that produce those effects. In this regard, a convective warming system typically includes a source of warmed pressurized air (also called a heater/blower unit, a forced air warming unit, a heater unit, etc.), a convective device, and a flexible conduit or air hose connecting the heater/blower unit with the convective device. Use of such a system for a particular type of warming requires delivery of warmed air through the convective device at parametric values that achieve a particular objective. The conditions by which a convective device such as a presently-designed thermal blanket produces thermal comfort in normothermic individuals at steady state are significantly different from those necessary to treat hypothermia. Typically the conditions for thermal comfort are met in a comfort warming system with a relatively low capacity heater/blower unit, while those in a therapeutic warming system are achieved with a relatively high capacity heater/blower unit. The different capacities have led to use of air hoses with different capacities, with those delivering air flow for thermal comfort typically having smaller diameters than those serving a therapeutic warming requirement. The result is a divergence of designs leading to installation of different air delivery infrastructures for therapeutic and comfort warming.
In some perioperative circumstances, the indiscriminate use of thermal blankets for comfort warming during the intraoperative and postoperative periods may expose the patient to several dangers. In particular, the use of a thermal blanket to produce comfort warming when therapeutic warming is indicated may significantly prolong the time necessary to produce normothermia. Thus it is sometimes advantageous and even safe to restrict the use of comfort warming to those settings where it is appropriate, which are principally in the preoperative period.
The application of warmed air to the limbs by a comfort warming system produces the sense of well-being that characterizes comfort warming because of the high density of thermoreceptors in the arms and legs. Warming the peripheral body regions produces a greater comfort response than thermal stimulation of the anterior or posterior abdominal and thoracic body regions. One surprising result of warming preoperatively by heating the limbs is that the increase of thermal energy content in the body's periphery prevents or reduces the core temperature drop caused by core-to-periphery redistribution. Thus, while warming the limbs preoperatively does not produce an increase in core body temperature, it does prevent that temperature from dropping once anesthesia is initiated. Warming the limbs preoperatively in order to prevent or delay a drop in core body temperature may be referred to as “prewarming.”
However, most presently-designed thermal blankets are designed to warm the anterior thoracic and/or abdominal regions of the body. They are not particularly well adapted to provide prewarming or comfort warming because of the low density of thermo-receptors in the central anterior region of the body. The comfort warming devices taught in PCT Publication No. WO 03/086500 are very effective for ambulatory patients put in control of the comfort warming parameters. However, these devices are not particularly well-suited to supine patients who are being prepared for surgery. In particular, they do not provide prewarming and/or comfort warming that is focused on the limbs.